Nitride-based III-V group compound semiconductors represented by the general formula InxGayAlzN (wherein, x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1) have controllable direct type band gap corresponding to from ultraviolet to red depending on the composition of a III group element, consequently, can be utilized as a material for a light emitting device of high efficiency ranging from ultraviolet to visible light. It is theoretically possible to manufacture an electronic device excellent in environment resistance utilizing a property as a semiconductor even at high temperatures under which conventional semiconductors cannot operate, due to larger band gap as compared with semiconductors such as GaAs and the like generally used up to now.
However, nitride-based III-V group compound semiconductors cannot easily perform crystal growth of a bulk single crystal, and free standing substrates of practically endurable nitride-based III-V group compound semiconductors are still under developing. Therefore, substrates widely used currently are sapphire and the like. Usually, methods of epitaxial growth such as a metal-organic chemical vapor deposition method (hereinafter, abbreviated as MOCVD method) are used.
However, a sapphire substrate has a lattice constant different significantly from that of a nitride-based III-V group compound semiconductor, resultantly, it is impossible to grow directly on this a crystal of a nitride-based III-V group compound semiconductor. Therefore, there is invented and usually used a method of growing amorphous GaN, AlN or the like once at lower temperature, relaxing lattice strain, and then, growing a crystal of a nitride-based III-V group compound semiconductor on this (Japanese Patent Application Laid-Open (JP-A) No. 63-188983). By this method, the quality of a crystal of a nitride-based III-V group compound semiconductor has increased dynamically.
However, since a discrepancy of lattice constant between a sapphire substrate and a crystal of a nitride-based III-V group compound semiconductor is not resolved, dislocation, this is a crystal defect, is still present at a density of as high as 109 to 1010 cm−2 in the crystal of a nitride-based III-V group compound semiconductor. This dislocation is a problem since it remarkably decreases the performance of an device such as life or the like.
Then, recently, as a method of reducing dislocation generating based on the discrepancy of lattice constant from sapphire, there is suggested a method in which on GaN having dislocation present in high density, a mask patterned with SiO2 and the like is formed, GaN is grown from a window portion of the mask and the mask is covered by lateral growth to obtain a flat GaN crystal, and there is also reported that dislocation density can be decreased to 107 cm−2 by blocking dislocation from the template using a mask (Appl. Phys. Lett. 71(18) 2637 (1997)).
On the other hand, as a method of obtaining a free standing GaN substrate, a method is reported in which on a sapphire substrate and the like, a GaN crystal is epitaxially grown, and sapphire and the like are removed by using etching or laser (Jpn. J. Appl. Phys. vol. 38, p. L217-219 (1999), JP-A No. 2000-129000).
However, this method has a problem that because of a difference in thermal expansion coefficient between sapphire and the like and GaN, warp occurs in a cooling step after growth, consequently, warp or crack remains on the resulted free standing substrate, further, a problem that dislocation density sufficiently reduced cannot be obtained.
As a method of solving these problems, there is suggested a method in which a metal thin film made of Ti and the like having a catalytic action promoting decomposition of GaN is formed on the surface of GaN, then, a thermal treatment is conducted on this under an atmosphere containing NH3, to form TiN in the form of mesh on GaN and simultaneously to form a void of reverse cone shape on the ground GaN in the mesh space, and GaN is laterally grown on this TiN, then, this is peeled by using mixed liquid of hydrofluoric acid and nitric acid to obtain a free standing substrate having a dislocation density reduced to about 107 cm−2 and little warp. (JP-A No. 2002-343728)
However, also this method has a problem that warp is not sufficient though it is decreased and nitriding of Ti and formation of a void in GaN are conducted simultaneously in a thermal treatment, consequently, control of the void proportion and adjustment of the degree of nitriding of Ti and the like are difficult and stable production of a substrate of low dislocation is difficult, a problem that mixed liquid of hydrofluoric acid and nitric acid is necessary for conducting peeling, and other problems.